Carbon-based fossil fuels, such as coal, petroleum and natural gas, are finite and non-renewable resources. At the current consumption rate, supplies of fossil fuels will be exhausted in the foreseeable future. At the meantime, burning fossil fuels has resulted in a rise in the concentration of carbon dioxide in the atmosphere, which is believed to have caused global climate change.
Biofuels are viable alternatives to fossil fuels for several reasons. Biofuels are renewable energy sources produced from biomass, a material derived from recently living organisms. Although biofuels are also carbon-based, they do not cause a significant net increase in atmospheric carbon dioxide levels because the carbon dioxide released during the fuel consumption is reabsorbed through new growth of the organisms.
Because transportation-related gasoline consumption represents the majority of all liquid fossil fuel use, supplementing or replacing gasoline with liquid biofuels is expected to reduce our reliance on fossil fuels and carbon dioxide production. Currently available liquid biofuels include ethanol and lipids. Ethanol is typically produced from crops rich in carbohydrates such as sugar and starch. Complex carbohydrates such as cellulose and hemicellulose can also be broken down into sugar, which can then be converted to ethanol by microorganisms. Lipids-based biofuels, also called biodiesels, are vegetable oils derived from vegetables such as corn, soybean, sunflower, and sorghum.
The energy benefit of using ethanol and lipid-based biofuels has, however, been called into question. Ethanol has lower energy content than gasoline such that more ethanol is required to provide the same energy output. More significantly, both ethanol and lipid productions are currently driven by fossil fuel. For example, the energy for producing ethanol includes running farm machinery and irrigation, transporting and grinding the crop, producing pesticides and fertilizer and fermenting and distilling ethanol. There have been concerns that the energy input for ethanol production may exceed the energy output from the combustion of ethanol. In addition, widespread production and use of ethanol and biodiesel will require constructing new distribution pipelines because neither is suitable for transportation using existing fuel-distribution infrastructure. Moreover, any large-scale development of crop-based fuels such as ethanol and traditional biodiesel will compete for the same resources as food production, and ultimately be limited by the amount of arable land.
Accordingly, there is a need for producing fuels from renewable sources, as well as overcoming the drawbacks of existing biofuels.